Determining Electric
Motor Load Factor
M
'
ost likely your operation's
motors account for a large
part of your monthly electrical bill. Make certain your motors are properly loaded. This
will increase their reliabiZity and
efjciciency-saving you money
and energy, and helping you
avoid potential problems. This
Technology Update enables
you to determine the load factor
and efficiency of your electric motors-information you can use to
identify motors that should be replaced with a different size or increased efficiency model.
Reasons to Determine Motor Loading
Most electric motors are designed to run at 50 to 100 percent of rated
load. Optimum efficiency is at 75 percent of rated load. Thus, a 10 horsepower (hp) motor has an acceptable load range of 5 to 10 hp; peak efficiency is at 7.5hp (see Figure 1).Surveys of installed motors indicate
that a large portion of motors in use are improperly loaded.
Underloaded motors, those loaded below 50 percent of rated load, are
ineffiaent and exhibit low power factor (see Figure 2). Power factor is a
measurement of the phase angle lag between electrical voltage and current, with 100%as optimum. (See Technology Update, Reducing Power
Factor Cost for more information.) Low power factor results in increased electrical distribution system losses. For this reason an increasing number of utilities charge a penalty for low power factor. Replacing
underloaded motors with correctly sized motors improves efficiency
and raises power factor.
I
Motor Part Load Efficient
Y
as a Functlon of % Full Load E ficiency
a
r
0
.-: s o %
G
i;i
u
m
8oo/c
0
J
--
Lad Ranger:
40%
LL
3
c
c
Q1
20%
$
p.
0%
20%
40%
60%
80%
e
Percent Full Load
0 - 1 P
1.5-5 hp
P
Toll-free Hotllne 1-800-872-3568
Bonnwille
POWER ADMINISTRATION
FAX 1-800-872-3882
.....lO..hP
. ?!:SP.
..........
------
15-25 hp
I
hP
75-100 hp
Electronic Bulletin Board 1-800-762-3319
Figure 2
rotates under its full rated load
and is usually about 3 to 5 percent slower than synchronous
speed. Most electric motors have
a full load RPM rating stamped
on their nameplate.
Motor Power Factor
%O%j
as a Function of % Full Load Amperage
Slip is characteristically linearly
proportional to load (see Figure
3). For example, a motor running
with 50 percent load factor has a
slip halfway between full load
and synchronous RPM. By using
a tachometer to measure actual
motor speed, it is possible to cald a t e motor loads.
i5%
45%
!k'o
i5%
kyo
k0/,
85%
1do%
Then calculate load using the
formula in Figure 5.
Percent Full Load Amperage
Overloaded motors can overheat
and lose efficiency. Many motors are designed with a service
factor that allows occasional
overloading. Service factor is a
multiplier that indicates how
much a motor can be overloaded. For example, a 10 horsepower motor with a 1.15service
factor can handle an 11.5 horsepower load for short periods of
time without incurring significant damage. Although many
motors have service factors of
1.15,running a motor above the
rated load reduces efficiency,
and if done frequently, reduces
service life.
If your operation uses equip
ment with motors that operate
for extended periods under 50
percent load, consider making
modifications. Sometimes motors are oversized because they
must accommodate peak conditions, such as when a pumping
system must satisfy occasionally
high demands. Options available to meet peak condition
needs include two-speed motors, adjustable-speed drives,
and management strategies that
First, determine fullload slip
and measured load slip. The
formulas are listed in Figure 4.
maintain motor loads within the
design range.
Determining if your motors are
properly loaded enables you to
make informed decisions about
when to replace motors and
which replacements to choose.
Measuring Motor Loads is relatively quick and easy when you
use the worksheet provided in
this Technology Update. You should
perform a motor load and efficiency analysis on all your major
working motors as part of your
preventative maintenance and energy conservation program.
"9
Motorlat
Rep acement
Annual Savings and
Simple Payback
The "Motor Load and Efficiency
Analysis" form (on page 4) provides a stepby-step process for
determining motor load and efficiency, and calculating the potential annual dollar savings
and resulting simple payback
I Figure 3
Percent Motor Slip
Determining Loads
A relatively simple way to determine the load on a working motor is by measuring "slip." Slip
refers to the difference between
the motor's synchronous speed
and loaded speed. Synchronous
RPM is the speed of the motofs
rotating electric field, usually
either 3,600,1,800,1,200,or 900
RPM. An unloaded motor rotates at approximatelyits synchronous speed. Full load FWM
is the speed that the motor
2
as a Furctlan of M
o
b Load
0
ND
Load
Full
Load
Load
full
load Slip = synchronous speed (RPM) - rated fullload speed (RPM)
load = motor HP x (measured running slip/full load slip)
measured
load Slip = synchronous speed (RPM)- measured load speed (RPM)
Example:
10 HP 1,800 RPM synchronous motor
Example:
full load speed = 1,740
full load slip = 60
-
synchronous speed = 1,800 RPM
measured load speed = 1,778 RPM
measured load slip = 22 RPM
nameplate) = 1,740 RPM
full load speed (f"
full load slip = 1,800 - 1,740 = 60 RPM
10 x 22/60 = 3.7 HP load, or 37%o hull load
measured load speed (bytachometer) = 1,778 RPM
measured load slip = 1,800 1,778 = 22 RPM
-
from replacing a significantly
oversized and underloaded motor with a more efficient, properly sized model. (Simple
payback is a measurement of
how long it will take for the efficiency savings to compensate for
the increased cost of the more efficient motor .)
We recommend that you survey
and test all motors operating over
1O
, OO hours per year. Using the
analysis results, divide your m e
tors into the following categories:
H Motors that are significantly
oversized and underloadedreplace with more efficient,
properly sized models at the
next opportunity, such as
scheduled plant downtime.
Motors that are moderately
oversized and underloadedreplace with more efficient,
properly sized models when
they fail.
Motors that are properly
sized but standard efficiency-replace most of these
with energy-efficient models
when they fail. The cost effectiveness of an energy-efficient
motor purchase depends on
the number of hours the motor is used, the price of electricity, and the price premium
of buying an energy-efficient
motor.
H The slip technique should not
be used for rewound motors
or motors operating at other
than design voltage since full
load RPM nameplate data is
no longer accurate.
H This analysis is designed for
motors operating under
steady load conditions. If
loads vary, you'll need to perform a separate analysis for
each portion of the load m e
to obtain annual energy savings.
4 This analysis may be inappro-
priate for motors driving conveyors or crushers, since
oversizing may be required
due to startup torque requirements, transient loads, or abnormal operating conditions.
4 This analysis requires use of a
tachometer to measure motor
RPM, and a Multimeter to
measure voltage and current.
Such monitoring is potentially hazardous and requires
proper equipment calibration;
it should only be performed
by trained personnel.
4 Several of the values used in
this analysis, including rated
full load RPM, measured
RPM, and current are subject
3
to rounding errors and variations. For example, full load
ratings are often rounded to
the nearest 5 RPM, and a specific motor may have an actual speed slightly different
than the nameplate value established for that model.
These differencescan lead to
significant variation and uncertainty in this analysis,
which should be considered
when evaluating results.
4 Many utility energy conser-
vation programs offer rebates
for the purchase of energy
efficient motors and other
industrial conservation activities. Contact your utility conservation department for
information.
Company
Plant
Datemime
Building
Dept.
Application
Motor Shop Number
Phase and HZ
Make
Frame size
Model/Type
Insulation class
Serial number
Efficiency rating
Service factor
NEMA torque type
Enclosure type
Temperature rise
A) Full load HP
B) Volts
N) Running slip (E-K)
C) Amperes
P) Per cent load (N/F)(~00%)
D) Full load speed
Q) HP Output (A)(P)(l 00%)
E) Sync. speed
R) kW output (cq(o.746)
2 pole = 3,60O,4 pole = 1,800, 6 pole = 1,200)
F) Full load slip (E-D)
S ) Eff. per cent (w~)(loo%)
T) kVA input (G)(H)(1.732)/(1000)
G) Average volts
U) Power factor (J)/(r)(i 00%)
H) Average amperes
v)
J) Average kW
W) $/Year operation (J)(L)(M)
K) Operating speed, RPM
X) $/Year losses (v)(L)(M)
L) Full load operating hours
Y)' Annual energy savings due to changeout with a h
high-efficiency motor (R)(L)(M)(lOO/S - 1WIEFF)
kW IOSSeS (J-R)
W) Average electricity price including demand charges Z)* Replacement motor cost
($/kWh)
Simple payback, years (Z)/(Y)
@
EFF Is the efflclency (%) of a mpiacement premium efflclency motor at the appropriate load factor.
!Costis the total cost of purchaslngand installing an optlmaiiy slzed, high-efficiency motor
4
p
V Electric Ideas Clearinghouseis a
comprehensive information
source for
and industrial energy users. It is operated
by the WashingtonState Enflu
Office and is part of the Electric
Ideas technology transfer program sponsored by participating
utilities and the Bonmille
Power Administration.
V Neither the United States nor the
Bonneville Power Administration, the state of Washington, the
WashingtonState Energy Office,
nor any of their contractors, subcontractors, or their employees
make any warranty, expressed or
implied, or assume any legal responsibility for the accuracy, completeness, or usefulness of any
information,apparatus, product,
or process disclosed within the
publication or through the Clearinghouse Bulletin Board System.
V Technology Update CH-17
'
I
Toll-free Hotline:
1-800-872-3568
Fax: 1-800-872-3882
Electronic Bulletin Board:
1-800-762-3319
DOYBP-1768
lune 1992
15C
Bibliography
McCoy, G., T.Litman, and J. Douglas. Energy-Efficient Electric Motor Selection Handbook. Prepared for the Bonneville Power Administration by the
Washington State Energy Office. DOE/BP-34623-3. October 1991.